Bispecific Antibodies
Bispecific antibodies (BsAbs) which have binding specificities for at least two different antigens have significant potential in a wide range of clinical applications as targeting agents for in vitro and in vivo immunodiagnosis and therapy, and for diagnostic immunoassays.
In the diagnostic areas, bispecific antibodies have been very useful in probing the functional properties of cell surface molecules and in defining the ability of the different Fc receptors to mediate cytotoxicity (Fanger et al., Crit. Rev. Immunol. 12:101–124 (1992)). Nolan et al., Biochem. Biophys. Acta. 1040:1–11 (1990) describe other diagnostic applications for BsAbs. In particular, BsAbs can be constructed to immobilize enzymes for use in enzyme immunoassays. To achieve this, one arm of the BsAb can be designed to bind to a specific epitope on the enzyme so that binding does not cause enzyme inhibition, the other arm of the BsAb binds to the immobilizing matrix ensuring a high enzyme density at the desired site. Examples of such diagnostic BsAbs include the rabbit anti-IgG/anti-ferritin BsAb described by Hammerling et al., J. Exp. Med. 128:1461–1473 (1968) which was used to locate surface antigens. BsAbs having binding specificities for horse radish peroxidase (HRP) as well as a hormone have also been developed. Another potential immunochemical application for BsAbs involves their use in two-site immunoassays. For example, two BsAbs are produced binding to two separate epitopes on the analyte protein—one BsAb binds the complex to an insoluble matrix, the other binds an indicator enzyme (see Nolan et al., supra).
Bispecific antibodies can also be used for in vitro or in vivo immunodiagnosis of various diseases such as cancer (Songsivilai et al., Clin. Exp. Immunol. 79:315 (1990)). To facilitate this diagnostic use of the BsAb, one arm of the BsAb can bind a tumor associated antigen and the other arm can bind a detectable marker such as a chelator which tightly binds a radionuclide. Using this approach, Le Doussal et al. made a BsAb useful for radioimmunodetection of colorectal and thryoid carcinomas which had one arm which bound a carcinoembryonic antigen (CEA) and another arm which bound diethylenetriaminepentacetic acid (DPTA). See Le Doussal et al., Int. J. Cancer Suppl. 7:58–62 (1992) and Le Doussal et al., J. Nucl. Med. 34:1662–1671 (1993). Stickney et al. similarly describe a strategy for detecting colorectal cancers expressing CEA using radioimmunodetection. These investigators describe a BsAb which binds CEA as well as hydroxyethylthiourea-benzyl-EDTA (EOTUBE). See Stickney et al., Cancer Res. 51:6650–6655 (1991).
Bispecific antibodies can also be used for human therapy in redirected cytotoxicity by providing one arm which binds a target (e.g. pathogen or tumor cell) and another arm which binds a cytotoxic trigger molecule, such as the T-cell receptor or the Fcγ receptor. Accordingly, bispecific antibodies can be used to direct a patient's cellular immune defense mechanisms specifically to the tumor cell or infectious agent. Using this strategy, it has been demonstrated that bispecific antibodies which bind to the FcγRIII (i.e. CD16) can mediate tumor cell killing by natural killer (NK) cell/large granular lymphocyte (LGL) cells in vitro and are effective in preventing tumor growth in vivo. Segal et al., Chem. Immunol. 47:179 (1989) and Segal et al., Biologic Therapy of Cancer 2(4) DeVita et al. eds. J. B. Lippincott, Philadelphia (1992) p. 1. Similarly, a bispecific antibody having one arm which binds FcγRIII and another which binds to the HER2 receptor has been developed for therapy of ovarian and breast tumors that overexpress the HER2 antigen. (Hseih-Ma et al. Cancer Research 52:6832–6839 (1992) and Weiner et al. Cancer Research 53:94–100 (1993)). Bispecific antibodies can also mediate killing by T cells. Normally, the bispecific antibodies link the CD3 complex on T cells to a tumor-associated antigen. A fully humanized F(ab′)2 BsAb consisting of anti-CD3 linked to anti-p185HER2 has been used to target T cells to kill tumor cells overexpressing the HER2 receptor. Shalaby et al., J. Exp. Med. 175(1):217 (1992). Bispecific antibodies have been tested in several early phase clinical trials with encouraging results. In one trial, 12 patients with lung, ovarian or breast cancer were treated with infusions of activated T-lymphocytes targeted with an anti-CD3/anti-tumor (MOC31) bispecific antibody. deLeij et al. Bispecific Antibodies and Targeted Cellular Cytotoxicity, Romet-Lemonne, Fanger and Segal Eds., Lienhart (1991) p. 249. The targeted cells induced considerable local lysis of tumor cells, a mild inflammatory reaction, but no toxic side effects or anti-mouse antibody responses. In a very preliminary trial of an anti-CD3/anti-CD19 bispecific antibody in a patient with B-cell malignancy, significant reduction in peripheral tumor cell counts was also achieved. Clark et al. Bispecific Antibodies and Targeted Cellular Cytotoxicity, Romet-Lemonne, Fanger and Segal Eds., Lienhart (1991) p. 243. See also Kroesen et al., Cancer Immunol. Immunother. 37:400–407 (1993), Kroesen et al., Br. J. Cancer 70:652–661 (1994) and Weiner et al., J. Immunol. 152:2385 (1994) concerning therapeutic applications for BsAbs.
Bispecific antibodies may also be used as fibrinolytic agents or vaccine adjuvants. Furthermore, these antibodies may be used in the treatment of infectious diseases (e.g. for targeting of effector cells to virally infected cells such as HIV or influenza virus or protozoa such as Toxoplasma gondii), used to deliver immunotoxins to tumor cells, or target immune complexes to cell surface receptors (see Fanger et al., supra).
Use of BsAbs has been effectively hindered by the difficulty of obtaining BsAbs in sufficient quantity and purity. Traditionally, bispecific antibodies were made using hybrid-hybridoma technology (Millstein and Cuello, Nature 305:537–539 (1983)). Because of the random assortment of immunoglobulin heavy and light chains, these hybridomas (quadromas) produce a potential mixture of 10 different antibody molecules, of which only one has the correct bispecific structure (see FIG. 1A). The purification of the correct molecule, which is usually done by affinity chromatography steps, is rather cumbersome, and the product yields are low. See, for example, (Smith, W., et al. (1992) Hybridoma 4:87–98; and Massimo, Y. S., et al. (1997) J. Immunol. Methods 201:57–66). Accordingly, techniques for the production of greater yields of BsAb have been developed. To achieve chemical coupling of antibody fragments, Brennan et al., Science 229:81 (1985) describe a procedure wherein intact antibodies are proteolytically cleaved to generate F(ab′)2 fragments. These fragments are reduced in the presence of the dithiol complexing agent sodium arsenite to stabilize vicinal dithiols and prevent intermolecular disulfide formation. The Fab′ fragments generated are then converted to thionitrobenzoate (TNB) derivatives. One of the Fab′-TNB derivatives is then reconverted to the Fab′-thiol by reduction with mercaptoethylamine and is mixed with an equimolar amount of the other Fab′-TNB derivative to form the BsAb. The BsAbs produced can be used as agents for the selective immobilization of enzymes.
Recent progress has facilitated the direct recovery of Fab′-SH fragments from E. coli, which can be chemically coupled to form bispecific antibodies. Shalaby et al., J. Exp. Med. 175:217–225 (1992) describe the production of a fully humanized BsAb F(ab′)2 molecule having one arm which binds p185HER2 and another arm which binds CD3. Each Fab′ fragment was separately secreted from E. coli, and subjected to directed chemical coupling in vitro to form the BsAb. The BsAb thus formed was able to bind to cells overexpressing the HER2 receptor and normal human T cells, as well as trigger the lytic activity of human cytotoxic lymphocytes against human breast tumor targets. See also Rodrigues et al., Int. J. Cancers (Suppl.) 7:45–50 (1992).
Various techniques for making and isolating BsAb fragments directly from recombinant cell cultures have also been described. For example, bispecific F(ab′)2 heterodimers have been produced using leucine zippers (Kostelny et al., J. Immunol. 148(5):1547–1553 (1992)). The leucine zipper peptides from the Fos and Jun proteins were linked to the Fab′ portions of anti-CD3 and anti-interleukin-2 receptor (IL-2R) antibodies by gene fusion. The antibody homodimers were reduced at the hinge region to form monomers and then reoxidized to form the antibody heterodimers. The BsAbs were found to be highly effective in recruiting cytotoxic T cells to lyse HuT-102 cells in vitro. The advent of the “diabody” technology described by Hollinger et al., PNAS (USA) 90:6444–6448 (1993) has provided an alternative mechanism for making BsAb fragments. The fragments comprise a heavy chain variable domain (VH) connected to a light chain variable domain (VL) by a linker which is too short to allow pairing between the two domains on the same chain. Accordingly, the VH and VL domains of one fragment are forced to pair with the complementary VL and VH domains of another fragment, thereby forming two antigen-binding sites. Another strategy for making BsAb fragments by the use of single chain Fv (sFv) dimers has also been reported. See Gruber et al. J. Immunol. 152: 5368 (1994). These researchers designed an antibody which comprised the VH and VL domains of an antibody directed against the T cell receptor joined by a 25 amino acid residue linker to the VH and VL domains of an anti-fluorescein antibody. The refolded molecule bound to fluorescein and the T cell receptor and redirected the lysis of human tumor cells that had fluorescein covalently linked to their surface.
It is apparent that several techniques for making bispecific antibody fragments which can be recovered directly from recombinant cell culture have been reported. However, full length BsAbs may be preferable to BsAb fragments for many clinical applications because of their likely longer serum half-life and possible effector functions.
Immunoadhesins
Immunoadhesins (Ia's) are antibody-like molecules which combine the binding domain of a protein such as a cell-surface receptor or a ligand (an “adhesin”) with the effector functions of an immunoglobulin constant domain. Immunoadhesins can possess many of the valuable chemical and biological properties of human antibodies. Since immunoadhesins can be constructed from a human protein sequence with a desired specificity linked to an appropriate human immunoglobulin hinge and constant domain (Fc) sequence, the binding specificity of interest can be achieved using entirely human components. Such immunoadhesins are minimally immunogenic to the patient, and are safe for chronic or repeated use.
Immunoadhesins reported in the literature include fusions of the T cell receptor (Gascoigne et al., Proc. Natl. Acad. Sci. USA 84:2936–2940 (1987)); CD4 (Capon et al., Nature 337:525–531 (1989); Traunecker et al., Nature 339:68–70 (1989); Zettmeissl et al., DNA Cell Biol. USA 9:347–353 (1990); and Byrn et al., Nature 344:667–670 (1990)); L-selectin or homing receptor (Watson et al., J. Cell. Biol. 110:2221–2229 (1990); and Watson et al., Nature 349:164–167 (1991)); CD44 (Aruffo et al., Cell 61:1303–1313 (1990)); CD28 and B7 (Linsley et al., J. Exp. Med. 173:721–730 (1991)); CTLA-4 (Lisley et al., J. Exp. Med. 174:561–569 (1991)); CD22 (Stamenkovic et al., Cell 66:1133–1144 (1991)); TNF receptor (Ashkenazi et al., Proc. Natl. Acad. Sci. USA 88:10535–10539 (1991); Lesslauer et al., Eur. J. Immunol. 27:2883–2886 (1991); and Peppel et al., J. Exp. Med. 174:1483–1489 (1991)); NP receptors (Bennett et al., J. Biol. Chem. 266:23060–23067 (1991)); inteferon γ receptor (Kurschner et al., J. Biol. Chem. 267:9354–9360 (1992)); 4-1BB (Chalupny et al., PNAS (USA) 89:10360–10364 (1992)) and IgE receptor α (Ridgway and Gorman, J. Cell. Biol. Vol. 115, Abstract No. 1448 (1991)).
Examples of immunoadhesins which have been described for therapeutic use include the CD4-IgG immunoadhesin for blocking the binding of HIV to cell-surface CD4. Data obtained from Phase I clinical trials in which CD4-IgG was administered to pregnant women just before delivery suggests that this immunoadhesin may be useful in the prevention of maternal-fetal transfer of HIV. Ashkenazi et al., Intern. Rev. Immunol. 10:219–227 (1993). An immunoadhesin which binds tumor necrosis factor (TNF) has also been developed. TNF is a proinflammatory cytokine which has been shown to be a major mediator of septic shock. Based on a mouse model of septic shock, a TNF receptor immunoadhesin has shown promise as a candidate for clinical use in treating septic shock (Ashkenazi et al., supra). Immunoadhesins also have non-therapeutic uses. For example, the L-selectin receptor immunoadhesin was used as an reagent for histochemical staining of peripheral lymph node high endothelial venules (HEV). This reagent was also used to isolate and characterize the L-selectin ligand (Ashkenazi et al., supra).
If the two arms of the immunoadhesin structure have different specificities, the immunoadhesin is called a “bispecific immunoadhesin” by analogy to bispecific antibodies. Dietsch et al., J. Immunol. Methods 162:123 (1993) describe such a bispecific immunoadhesin combining the extracellular domains of the adhesion molecules, E-selectin and P-selectin. Binding studies indicated that the bispecific immunoglobulin fusion protein so formed had an enhanced ability to bind to a myeloid cell line compared to the monospecific immunoadhesins from which it was derived.
Antibody-Immunoadhesin Chimeras
Antibody-immunoadhesin (Ab/Ia) chimeras have also been described in the literature. These molecules combine the binding region of an immunoadhesin with the binding domain of an antibody.
Berg et al., PNAS (USA) 88:4723–4727 (1991) made a bispecific antibody-immunoadhesin chimera which was derived from murine CD4-IgG. These workers constructed a tetrameric molecule having two arms. One arm was composed of CD4 fused with an antibody heavy-chain constant domain along with a CD4 fusion with an antibody light-chain constant domain. The other arm was composed of a complete heavy-chain of an anti-CD3 antibody along with a complete light-chain of the same antibody. By virtue of the CD4-IgG arm, this bispecific molecule binds to CD3 on the surface of cytotoxic T cells. The juxtaposition of the cytotoxic cells and HIV-infected cells results in specific killing of the latter cells.
While Berg et al. supra describe a bispecific molecule that was tetrameric in structure, it is possible to produce a trimeric hybrid molecule that contains only one CD4-IgG fusion. See Chamow et al., J. Immunol. 153:4268 (1994). The first arm of this construct is formed by a humanized anti-CD3 κ light chain and a humanized anti-CD3 γ heavy chain. The second arm is a CD4-IgG immunoadhesin which combines part of the extracellular domain of CD4 responsible for gp120 binding with the Fc domain of IgG. The resultant Ab/Ia chimera mediated killing of HIV-infected cells using either pure cytotoxic T cell preparations or whole peripheral blood lymphocyte (PBL) fractions that additionally included Fc receptor-bearing large granular lymphocyte effector cells.
In the manufacture of the multispecific antibody heteromultimers, it is desirable to increase the yields of the desired heteromultimer over the homomultimer(s). The current method of choice for obtaining Fc-containing BsAb remains the hybrid hybridoma, in which two antibodies are coexpressed (Milstein and Cuello, Nature 305:537–540 (1983)).
In hybrid hybridomas, heavy (H) chains typically form homodimers as well as the desired heterodimers. Additionally, light (L) chains frequently mispair with non-cognate heavy chains. Hence, coexpression of two antibodies may produce up to ten heavy and light chain pairings (Suresh, M. R., et al. Methods Enzymol. 121:210–228 (1986)). These unwanted chain pairings compromise the yield of the BsAb and inevitably impose significant, and sometimes insurmountable, purification challenges (Smith, et al. (1992) supra; and Massimo, et al. (1997) supra).
Antibody heavy chains have previously been engineered to drive heterodimerization by introducing sterically complementary mutations in multimerization domains at the CH3 domain interface (Ridgway et al. Protein Eng. 9:617–621 (1996)) and optimization by phage display as described herein. Chains containing the modified CH3 domains yield up to approximately 90% heterodimer as judged by formation of an antibody/immunoadhesin hybrid (Ab/Ia). Heterodimerized heavy chains may still mispair with the non-cognate light chain, thus hampering recovery of the BsAb of interest.